1. Field of the Invention
The present invention relates to a complex coefficient transversal filter and a complex frequency converter. More particularly, the present invention relates to a complex coefficient transversal filter using a surface acoustic wave (SAW) filter suitable for implementing a high-frequency complex coefficient filter for a high-performance radio communication terminal and a complex frequency converter using the complex coefficient transversal filter.
2. Description of the Related Art
A front end of a receiver performs frequency conversion on a radio frequency (RF) signal and selects a desired signal. The front end may be a heterodyne system using an intermediate frequency (IF) signal, a zero-IF system (a direct conversion system) which converts an RF signal into a baseband signal, and a low-IF system using an image rejection mixer which suppresses an image signal with a mixer.
[Heterodyne System]
In the heterodyne system, in order to prevent an image signal interference, the IF frequency is designed to be increased to increase a frequency difference between a target RF signal and an image frequency signal and to suppress the image frequency signal with an RF filter. For example, in a full duplex transceiver where a receiver and a transmitter operate simultaneously, an image of a transmission signal when a transmission local signal and a receptional local signal is commonly used needs to be suppressed, and large interference from an outside band of a system frequency band, which cannot be suppressed with an RF filter needs to be prevented. Therefore, different IF frequencies are used for different radio communication systems. In such a multi-mode transceiver, since separate IF filters are needed in order to provide different channel bandwidths for different modes, a problem exists in that a circuit size increases greatly.
[Zero-IF System]
The zero-IF system is suitable for a small-sized system because an IF filter can be formed in an integrated chip (IC). However, in the zero-IF system, since the RF signal is converted into a baseband signal, an IM2 problem exists, that is, a problem of second-order inter-modulation distortion caused by DC offset after frequency conversion and nonlinearity of a mixer. In turn, a problem of deterioration in an error vector magnitude (EVM) characteristic exists. The deterioration in the EVM characteristic is caused by a signal of a mixer and a signal generated from a local signal not being perpendicular to each other. In order to solve the problem, a circuit has been developed which can reduce an amplitude error and a phase error between I and Q components of the local signal (hereinafter, simply referred to as “between I and Q”) and reduce an error between transistors of the mixer. In addition, various techniques have been developed to compensate for errors between I and Q by converting a complex baseband signal into a digital signal and performing a digital signal process.
However, due to incompleteness of an analog circuit, there is a limitation to improve the errors between I and Q. For example, in multi-level modulation, a deterioration caused by interference between symbols exists, and in an orthogonal frequency division multiplexing (OFDM), a problem of interference between carriers exists. In a multiple-input multiple-output (MIMO) system, which is a type of communication system in a wireless LAN due to the incompleteness of the analog circuit, a limitation to increase communication speed exists.
[Low-IF System]
In the low-IF system, the image rejection mixer is used for the frequency conversion process to suppress a signal whose frequency is located on the opposite side of a frequency of the local signal with respect to a frequency of the target signal. The low-IF system can suppress the image frequency signal irrespective of the frequency characteristics of the RF and IF filters. Since an image suppression ratio does not depend on the characteristic of the RF filter, the IF filter having a shape characteristic is not required, and an IF frequency can be lowered. In addition, a frequency interval between the target frequency signal and the image frequency signal becomes double the IF frequency. Therefore, when the IF frequency is equal to the channel interval, the image frequency of the target channel corresponds to an adjacent channel next to the target channel.
Required specifications of a radio communication system using a down converter can be attained when a specification for blocking of an image frequency signal having a frequency twice as large as the IF frequency is below the image suppression ratio. In the low-IF system, since the IF frequency is low, an IF filter can be constructed with an active filter. In addition, the low-IF system can be constructed as a small-sized system formed in an IC. In addition, since different IF frequencies are required for different radio communication systems, the IF frequency can be commonly used for a multi-mode transceiver. Although the frequency can be fixed, the frequency band cannot be fixed. However, in such a gmC filter, a filter characteristic can be implemented in a variable manner so that a multi-mode system can be implemented without a plurality of filters.
However, in a conventional low-IF system, the image suppression ratio is about 30 dB (see Phillips Datasheets SA1920 and SA1921), which is too low. Due to the low image suppression ratio, the low-IF system can be used in radio communication systems in which poor blocking of the image frequency signal is tolerable. However, the conventional low-IF system cannot be used for a system requiring an image suppression ratio exceeding 30 dB.
Accordingly, there is a need for an improved complex frequency converter comprising a complex coefficient transversal filter that obtains an improved image suppression characteristic.